U.S. patent number 4,781,195 [Application Number 07/127,742] was granted by the patent office on 1988-11-01 for blood monitoring apparatus and methods with amplifier input dark current correction.
This patent grant is currently assigned to The BOC Group, Inc.. Invention is credited to Alan D. Martin.
United States Patent |
4,781,195 |
Martin |
November 1, 1988 |
Blood monitoring apparatus and methods with amplifier input dark
current correction
Abstract
In photoelectric apparatus such as a pulse oximeter for
monitoring a parameter of the blood in a living organism, a dark
current correction signal is applied to the input at the
preamplifier. The correction signal is substantially equal in
magnitude but opposite in sense to the photodetector output during
dark intervals when the illumination means of the apparatus is
disabled. Because dark current correction is accomplished at the
input of the preamplifier, the preamplifier has substantially
increased resistance to overloading caused by ambient light and
hence may have higher gain.
Inventors: |
Martin; Alan D. (Boulder,
CO) |
Assignee: |
The BOC Group, Inc. (Montvale,
NJ)
|
Family
ID: |
22431707 |
Appl.
No.: |
07/127,742 |
Filed: |
December 2, 1987 |
Current U.S.
Class: |
600/336;
356/41 |
Current CPC
Class: |
A61B
5/02416 (20130101) |
Current International
Class: |
A61B
5/024 (20060101); A61B 005/00 (); G01N
033/49 () |
Field of
Search: |
;128/633,664,665,667
;356/41 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Cohan; Alan
Attorney, Agent or Firm: Cassett; Larry R. Rathbun; Roger
M.
Claims
I claim:
1. Apparatus for monitoring a parameter of the blood in a body
structure comprising:
(a) photodetector means for producing a photodetector output signal
related to the amount of light impinging upon said photodetector
means;
(b) preamplification means having an input node for providing an
amplified signal related to a signal supplied to said input node,
said input node being connected to to said photodetector means for
receipt of said photodetector output signal;
(c) illumination means for emitting light and directing the emitted
light through the body structure so that emitted light transmitted
through said body structure will impinge upon said photodetector
means;
(d) means for periodically interrupting operation of said
illumination means to provide dark intervals, whereby the
photodetector output signal during each said dark interval will be
a dark interval photodetector output indicative of ambient light
impinging on said photodetector means;
(e) correction means for applying to said input node of said
preamplification means a correction signal substantially equal in
magnitude but opposite in sense to the dark interval photodetector
output signal whereby said amplified signal will be substantially
corrected for the effects of ambient light on said photodetector
means; and
(f) interpretation means for determining said parameter of the
blood from said corrected, amplified signal.
2. Apparatus as claimed in claim 1 wherein said correction means
includes means for determining the photodetector output signal
prevailing during each said dark interval by monitoring said
amplified signal.
3. Apparatus as claimed in claim 2 wherein said correction means
includes feedback means for adjusting said correction signal during
each dark interval so as to bring said amplified signal
substantially to a predetermined null value and then maintaining
said correction signal as so adjusted until the next dark
interval.
4. Apparatus as claimed in claim 3 wherein said feedback means
includes means for integrating said amplified signal during each
said dark interval and means for providing said correction signal
during each period between dark intervals responsive to the
integrated amplified signal accumulated during the last previous
dark interval.
5. Apparatus as claimed in claim 1, wherein said photodetector
means includes means for providing said photodetector output signal
as a photodetector current such that the magnitude of said
photodetector current is directly related to the amount of light
impinging upon said photodetector means, said correction means
including means for applying said correction signal as a current
substantially equal but opposite to the photodetector current
prevailing during the last preceding dark interval.
6. Apparatus as claimed in claim 5 wherein said preamplifier means
includes a transresistance amplifier stage incorporating an
operational amplifier defining said input node, an output node and
a feedback resistor connected between said input and output
nodes.
7. Apparatus as claimed in claim 6 wherein said correction means
includes integrator means having an input and an output for
integrating a signal applied to said integrator input and providing
an integrator output voltage proportional to said integrated
signal, integrator switching means for connecting said integrator
input to said output node of said preamplification means only
during said dark intervals, and current generator means for
generating said correction current in response to said integrator
output voltage.
8. Apparatus as claimed in claim 6 wherein said current generator
means includes an inverter operational amplifier having inverter
input and output nodes, an inverter feedback resistor connected
across said inverter input and output nodes, an inverter input
resistor connected between said inverter input node and said
integrator output node, and a proportioning resistor connected
between said inverter output node and said input node of said
preamplification means.
9. Apparatus as claimed in claim 7 wherein said current generator
means includes a correction current circuit branch connected
between said input node of said preamplification means and a source
of a predetermined bias voltage, and means for providing an
impedance in said correction current circuit branch such that said
impedance depends upon said integrator output voltage.
10. Apparatus as claimed in claim 9 wherein said current generator
means includes a pair of resistors connected in series between said
integrator output node and a source of predetermined bias voltage,
said resistances defining therebetween a circuit node, said means
for providing an impedance being responsive to the voltage at said
circuit node.
11. Apparatus as claimed in claim 10 wherein said means for
providing an impedance includes a transistor having a control input
connected to said circuit node.
12. Apparatus as claimed in claim 7 wherein said means for
periodically interrupting operation of said illumination means
includes a timing means and wherein said integrator switching means
is connected to said timing means and responsive thereto.
13. Apparatus as claimed in claim 1 wherein said illumination means
includes means for emitting light at a plurality of wavelengths,
the apparatus further includes sequence control means for
controlling said illumination means to emit light of different
wavelengths in alternating sequence at times other than said dark
intervals, and wherein said interpretation means includes means for
sampling said corrected, amplified signal in sequence correlated
with said alternating sequence of wavelengths and interpreting
different samples of said amplified signal as representing
transmissivity of the body structure at different wavelengths.
14. A method of monitoring a parameter of the blood in a body
structure comprising the steps of:
(a) producing a photodetector output signal related to the amount
of light impinging upon a photodetector means and providing said
photodetector output signal to an input of a preamplifier;
(b) amplifying the signal applied to said input of said
preamplifier so as to provide an amplified signal;
(c) illuminating the body structure by emitting light and directing
the emitted light through the body structure so that emitted light
transmitted through said body structure impinges upon said
photodetector;
(d) periodically interrupting said illuminating step to provide
dark intervals, whereby the photodetector output signal during each
said dark interval will be a dark interval photodetector output
indicative of ambient light impinging on said photodetector means;
and
(e) subtracting a correction signal from said photodetector output
signal before said photodetector output signal is amplified in step
(b), the magnitude of said correction signal being substantially
equal to the magnitude of said dark interval photodetector output
signal, whereby said amplified signal will be substantially
corrected for the effect of ambient light; and
(f) determining said parameter of the blood from said corrected
amplified signal.
Description
BACKGROUND OF THE INVENTION
The present invention relates to apparatus and methods for
monitoring a parameter in the blood of a living organism.
Certain constituents in the blood affect the absorption of light at
various wavelengths by the blood. For example, oxygen in the blood
binds to hemoglobin to form oxyhemoglobin. Oxyhemoglobin absorbs
light more strongly in the infrared region than in the red region,
whereas hemoglobin exhibits the reverse behavior. Therefore, highly
oxygenated blood with a high concentration of oxyhemoglobin and a
low concentration of hemoglobin will tend to have a high ratio of
optical transmissivity in the red region to optical transmissivity
in the infrared region. The ratio of transmissivities of the blood
at red and infrared wavelengths can be employed as a measure of
oxygen saturation.
This principle has been used heretofore in oximeters for monitoring
oxygen saturation of the blood in the body of a living organism as,
for example, in patients undergoing surgery. As disclosed in U.S.
Pat. No. 4,407,290, oximeters for this purpose may include red
light and infrared light emitting diodes together with a
photodetector. The diodes and photodetector typically are
incorporated in a probe arranged to fit on a body structure such as
an earlobe or a fingertip, so that light from the diodes is
transmitted through the body structure to the photodetector. The
infrared and red light emitting diodes are switched on and off in
alternating sequence at a switching frequency far greater than the
pulse frequency. The signal produced by the photodetector includes
alternating portions representing red and infrared light passing
through the body structure. These alternating portions are
amplified and then segregated by sampling devices operating in
synchronism with the red/infrared switching, so as to provide
separate signals on separate channels representing the red and
infrared light transmission of the body structure. After low-pass
filtering to remove signal components at or above the switching
frequency, each of the separate signals represents a plot of
optical transmissivity of the body structure at a particular
wavelength versus time.
Because the volume of blood in the body structure varies with the
pulsatile flow of blood in the body, each such signal includes an
AC component caused only by optical absorption by the blood and
varying at the pulse frequency or heart rate of the organism. Each
such signal also includes an invariant or DC component related to
other absorption, such as absorption by tissues other than blood in
the body structure. According to well known mathematical formulae,
set forth in said U.S. Pat. No. 4,407,290, the oxygen saturation in
the blood can be derived from the magnitudes of the AC and DC
components of these signals.
As also set forth in the '290 patent, the same general arrangement
can be employed to monitor constituents of the blood other than
oxygen such as carbon dioxide, carbon monoxide (as
carboxyhemoglobin) and/or blood glucose, provided that the other
constituents have some effect on the optical properties of the
blood. Also, information concerning the pulse of the patient can be
obtained from the AC signal components. As used in this disclosure,
the term "parameter of the blood" includes the level of any
constitutent and also includes parameters related to the pulse,
such as the pulse rate and the occurrence or non-occurrence of
pulses.
Measurement apparatus and methods of this type have been widely
adopted in the medical profession. However, such apparatus and
methods have been subject to interference from ambient light
falling on the photodetector. The apparatus has been provided with
circuits for cancelling components caused by ambient light. These
circuits operate by obtaining a "dark current" signal representing
the amplified photodetector signal during intervals when both of
the light emitting diodes are off and hence all of the light
falling on the photodetector represents ambient light. The dark
current signal value is used to cancel the ambient light component
in the amplified signals representing infrared and red light.
This approach provides only a partial solution to the ambient light
interference problem. The ambient light impinging upon the
photodetector may be far stronger than the light transmitted
through the patient's body. Accordingly, components of the
photodetector signal caused by ambient light may be far larger than
the useful photodetector signal components representing light
transmitted through the body structure. The ambient light
components can overload the first amplifier in the system, commonly
referred to as the preamplifier. To avoid such overloading, the
gain of the preamplifier has been limited heretofore. The limited
gain available in the preamplifier may result in a loss of
sensitivity in the instrument as a whole and may require greater
gain in subsequent stages used to amplify various portions of the
signal.
The conventional preamplifier utilized heretofore incorporates an
operational amplifier having inverting and non-inverting input
nodes and an output node. The non-inverting input node may be
grounded. The photodetector signal, typically a current from a
photodiode operating in a photoamperic mode, is connected to the
inverting input node of the operational amplifier. A feedback
resistor is connected between the inverting input node and the
output node. In this "transresistance" amplification arrangement,
the operational amplifier creates a voltage at the output node
opposite in sense to the voltage at the inverting input node. The
opposite sense voltage causes a current flow through the feedback
resistor opposite in sense to the current flow applied by the
photodetector. The preamplifier comes to equilibrium when the
current flow out of the inverting input node through the feedback
resistor exactly balances the current flow into the inverting input
node through the photodetector. The gain or ratio of output node
voltage to incoming signal is proportional to the value of the
feedback resistor. The greater the value of the feedback resistor,
the greater the opposite sense voltage at the output node must be
to achieve balance.
In the typical dark current cancellation circuitry utilized
heretofore, the output node of the preamplifier is connected to a
first side of a capacitor, whereas the second side of the capacitor
is connected to the downstream signal processing equipment. A
controllable switch is connected between the second side of the
capacitor and ground. The switch is closed only when the light
emitting diodes are off, i.e., only during dark intervals. During
each dark interval the preamplifier output node voltage represents
only the component caused by the ambient light. With the second
side of the capacitor grounded, the charge on the capacitor
accumulates until the voltage across the capacitor is equal to this
voltage. When the dark interval ends, the switch is opened, leaving
the charged capacitor connected between the amplifier output node
and the downstream signal processing apparatus. Therefore, the
voltage applied to the signal processing apparatus will be the
preamplifier output voltage less the voltage across the capacitor,
i.e., the preamplifier output voltage less the voltage component
caused by ambient light. So long as changes in ambient light levels
between successive dark intervals are relatively small, this
arrangement should theoretically provide good cancellation of the
signal components caused by the ambient light.
However, the dark current cancellation afforded in this arrangement
does not alleviate the problem of preamplifier overloading. Thus,
the operational amplifier must still provide sufficient voltage at
the output node so that the current through the feedback resistor
completely balances both the useful and ambient-light components of
the signal applied to the input node. The value of the feedback
resistor, and hence the gain of the preamplifier must be limited to
avoid exceeding the capacity of the operational amplifier.
Additionally, the operational amplifier is connected directly to a
significant capacitive load. Depending upon the design of the
particular operational amplifier, the capacitive load may induce
instabilities in the operational amplifier.
Accordingly, there have been needs for further improvements in the
blood parameter measuring apparatus, and specifically in the
ambient light cancellation arrangements used therein.
SUMMARY OF THE INVENTION
One aspect of the present invention incorporates the realization
that the problems caused by ambient light can be substantially
alleviated by correcting for dark current upstream of the
preamplifier or first amplification stage. In preferred apparatus
and methods according to this aspect of the invention, a correction
signal substantially equal in magnitude but opposite in sense to
the component of the photodetector signal caused by ambient light
is applied to the input node of the preamplifier. Because the
correction signal is applied to the input node, it effectively
counteracts the ambient light component in the photodetector signal
before that component has any effect on the preamplifier.
Therefore, the components in the photodetector signal caused by
ambient light cannot cause overloading of the preamplifier. For the
same reason, the gain of the preamplifier need not be limited to
avoid such overloading. This aspect of the present invention thus
provides improved apparatus and methods for monitoring a parameter
of the blood in a body structure.
Apparatus according to this aspect of the present invention
preferably includes photodetector means for detecting light and
producing a photodetector output signal related to the amount of
light impinging upon the photodetector means. The apparatus also
preferably includes preamplification means having an input node for
providing an amplified signal related to the signal applied to the
input node. The input node is connected to the photodetector means
for receipt of the photodetector output signal. Illumination means
preferably are provided for emitting light and directing the
emitted light through the body structure so that the emitted light
transmitted through the body structure will impinge upon the
photodetector means. Timing means are provided for periodically
interrupting the operation of the illumination means to provide
dark intervals. Thus, during each dark interval the photodetector
output signal will be a dark interval photodetector output signal
indicative of the ambient light impinging on the photodetector
means. Correction means are provided for applying to the input node
of the preamplification means a correction signal substantially
equal in magnitude but opposite in sign to the dark interval
photodetector output signal prevailing during the preceding dark
interval. Thus, during the time periods between dark intervals,
while the illumination means is operating, the net signal supplied
to the input node will be the photodetector signal less the
correction signal. In effect, the ambient light component is
subtracted out of the photodetector signal before the photodetector
signal ever passes into the input node of the preamplifier. As the
input signal to the preamplifier means is already substantially
corrected for the effect of ambient light on the photodetector
means, the amplified signal from the preamplifier means will be
substantially corrected for ambient light effects.
Preferably, the correction means includes means for determining the
photodetector output signal prevailing during each dark interval by
monitoring the amplified signal from the preamplification means.
Thus, the correction means may include feedback loop means for
applying a correction signal during each dark interval, adjusting
the correction signal during the dark interval until the amplified
signal goes to zero and then maintaining the correction signal at
the value established by this adjustment until the next dark
interval. Thus, the correction means may include means for
integrating the amplified signal during each dark interval and
means for providing the correction signal during periods between
the dark intervals responsive to the integrated, amplified signal
accumulated during the last previous dark interval. Typically, the
photodetector means is arranged to provide the photodetector output
signal as a photodetector current such that the magnitude of this
current is directly related to the amount of light impinging on the
photodetector means. The correction means may thus include means
for applying the correction signal to the preamplifier input node
as a current substantially equal but opposite in sense to the
photodetector current prevailing during the last preceding dark
interval.
The present invention also includes methods of monitoring a
parameter of the blood in a living subject. Preferred methods
according to this aspect of the invention include steps similar to
the function discussed above in connection with the apparatus. In
preferred methods according to this aspect of the invention, the
photodetector output signal or current is determined during each
dark interval and a corresponding but opposite correction signal or
current is applied to the input node of the preamplifier means
until the next succeeding dark interval. Methods according to this
aspect of the invention afford advantages similar to those achieved
with the apparatus.
These and other objects, features and advantages of the present
invention will be more readily apparent from the detailed
description of the preferred embodiments set forth below taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic, partially block-form diagram of apparatus
according to one embodiment of the invention.
FIG. 2 is a fragmentary schematic diagram showing a portion of
apparatus according to a further embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Apparatus according to one embodiment of the present invention
includes a probe 10 incorporating a clip 12 arranged to engage a
body structure such as finger 14. The probe also includes a red
light emitting diode 16 and an infrared light emitting diode 18
mounted to clip 12, together with a photodetector or photodiode 20
also mounted to clip 12. The light emitting diodes or "LED's" and
photodiode are arranged so that light emitted by the LED's will
pass through the finger 14 and impinge upon the photodiode. A red
LED drive 22 and infrared or "IR" LED drive 24 are connected to
LED's 16 and 18 respectively. A timing unit 26 is arranged to
actuate LED drives 22 and 24, and hence LED's 16 and 18, according
to a predetermined alternating sequence interspersed with dark
intervals. During each such dark interval, the timing unit 26
deactivates the LED drives and hence deactivates both LED's. Thus,
the LED drives and LED's provide alternating red and infrared
illumination, whereas the timing unit periodically interrupts this
illumination to provide the dark intervals.
Photodiode or photodetector 20 has a reference node 30 connected to
ground and an output node 32. A front end amplifier or
preamplification means 34 is also provided. Preamplification means
34 includes an operational amplifier 36 defining an inverting input
node 38, an output node 40 and a non-inverting input node 42
connected to ground. Node 38 is an "inverting" node in the sense
that amplifier 36 tends to produce a voltage at output node 40
opposite in sense to voltage at node 38. A feedback resistor 44 is
connected between inverting node 38 and output node 40.
photodetector output node 32 is connected to the inverting input
node 38 of operational amplifier 36.
The output node 40 of the preamplifier is connected to a sampling
switch 46, which in turn is connected to a red signal processing
channel 48 and an IR signal processing channel 50. Sampling switch
46 is controlled by timing unit 26 so that switch 46 operates in
synchronism with the predetermined sequence of red and infrared
emission from LED's 16 and 18. Thus, switch 46 samples the
amplifier output signal at preamplifier output node 40 and provides
a sequence of samples to each signal processing channel. While LED
16 is providing red light, the amplified signal obtained from
preamplifier 34 is routed through switch 46 to red signal
processing channel 48. Conversely, when infrared light is being
emitted by diode 18, the amplified signal is routed to IR signal
processing channel 50. During dark intervals, while neither diode
is operative, the amplified output signal is not routed to either
signal processing channel.
Each of signal processing channels 48 and 50 may include generally
conventional elements for converting the periodic signal samples
supplied through switch 46 into a substantially continuous,
smoothed signal, eliminating spurious components resulting from the
switching process itself and determining the AC and DC components
of the smoothed signal. For example, each signal processing channel
may include a first low pass filter having its input connected to
switch 46. The first low pass filter typically has a top cutoff
frequency of about 10 Hz, and is arranged to attenuate signals
above that frequency. The output of the first low pass filter is
connected directly to a first analog to digital converter, and also
to the input of a high pass filter. The high pass filter is
arranged to attentuate signals below about 0.5 Hz. The output of
the high pass filter may be connected to the input of an amplifier
within the signal processing channel, and the output of this
amplifier may be connected to a further low pass filter also having
a top cutoff frequency of about 10 Hz. The output of this further
low pass filter is connected to the input of a sample and hold
unit, and the output of the sample and hold unit in turn is
connected to a second analog to digital converter. A microprocessor
52 is connected to both signal processing channels 48 and 50, the
microprocessor being arranged to receive digital values from the
first and second analog to digital converter of each channel. The
microprocessor is also connected to a display unit 54.
Output node 40 of preamplifier means 34 is also connected to the
input side of an integrator switch 56. Switch 56 is controlled by
timing unit 26. The output side of integrator switch 56 is
connected through an integrator input resistor 58 to an input node
60 of an integrator 62. The integrator includes an integrator
operational amplifier 64 having an inverting input connected to
integrator input node 60, a non-inverting input node connected to
ground and an output connected to the output node 66 of the
integrator. The integrator also includes a capacitor 68 connected
between integrator input node 60 and integrator output node 66.
Integrator 62 is arranged to provide, at node 66, a voltage
directly related to the integral of the voltage applied to node
60.
Inverter means 68 includes an operational amplifier 70 defining an
inverting input node 72, a noninverting input node connected to
ground and an output node 74. Inverter input node 72 is connected
through an inverter input resistor 76 to the output node 66 of
integrator 62, and an inverter feedback resistor 78 is connected
between inverter input node 72 and inverter output node 74.
Inverter output node 74 is connected through a proportioning
resistor 80 to the input node 38 of preamplifier means 34.
In operation, timing unit 26 actuates LED drives 22 and 24 and
LED's 16 and 18 alternately, and periodically interrupts operation
of the LED's and LED drives to provide dark intervals during which
neither LED is illuminated. During each such dark interval, timing
unit 26 causes switch 56 to close thereby connecting preamplifier
means output node 40 through resistor 58 to integrator input node
60. During a dark interval, only the ambient light impinges upon
photodiode 20. As the current produced by photodiode 20 is directly
related to the amount of light impinging on the photodiode, the
current flowing out of the photodiode output node 32 at this time
is directly related to the amount of ambient light. The current
from the diode reaching preamplifier means input node 38 tends to
cause operational amplifier 36 to swing the voltage at preamplifier
output node 40 in the negative direction. This negative voltage is
applied to the input node 60 of integrator 62, and hence causes
integrator 62 to provide a positive voltage at integrator output
node 66. This positive voltage at input node 66 increases in
magnitude continually while the voltage at preamplifier output node
40 is negative with respect to ground.
Inverter means 68 provides a negative voltage with respect to
ground at output node 74 responsive to the positive voltage at the
integrator output node 66, the negative voltage at node 74 being
directly proportional to the positive voltage at integrator output
node 66. Thus, the negative voltage at node 74 will progressivly
increase while the voltage at preamplifier output node 40 remains
negative. There will be a progressively increasing current flow
towards node 74 and hence away from preamplifier input node 38.
This progressively increasing current flow tends to counteract the
current flowing towards the preamplifier input node from the
photodetector.
Stated another way, a correction current is applied through the
circuit branch leading through resistor 80, and the direction of
this correction current is opposite to the direction of the dark
current from photodector 20. So long as the dark current from
photodector 20 exceeds the correction current, the output of
preamplification means 34 at node 40 will be negative, and hence
the integrator output voltage at 66, and the magnitude of the
negative voltage at inverter output node 74 and the magnitude of
the correction current through resistor 80 will continue to grow.
However, when the magnitude of the correction current is equal to
the magnitude of the dark current from diode 20, there will be no
net current flow into preamplification means input node 38.
Accordingly, the voltage at preamplification means output node 40
and hence at integrator input node 60, will go to zero or ground
potential. The integrator output voltage at node 66 will then
stabilize and remain unchanged, as will the negative voltage at
node 74 and hence the correction current through resistor 80. Thus,
the system reaches equilibrium when the correction current through
resistor 80 equals the dark current from diode 20. The component
values are selected so that the system comes substantially to
equilibrium before the end of the dark interval.
Before the end of the dark interval, but after the correction
current has substantially reached equilibrium, timing unit 26
actuates switch 56 to open and hence to isolate the input node 60
of integrator 62. Once switch 56 is opened and the integrator input
is isolated, the integrator output remains substantially constant
and the inverter output and correction current through resistor 80
likewise remain substantially constant. This condition prevails
until the next dark interval, whereupon timing unit 26 closes
switch 56 once again and the same cycle of operations is repeated
to reset the correction current. Therefore, between dark intervals,
the system applies a correction current throught resistor 80
substantially equal in magnitude but opposite in sense to the
photodetector output current prevailing during the immediately
preceding dark interval.
Between the dark intervals, timing unit 26 actuates the LED drives
and hence LED's 16 and 18 to emit alternating bursts of red and
infrared light. Some of the light from the LED's will be
transmitted through the patient's body structure or fingertip 14 to
photodiode 20. Thus, the signal from photodiode 20 during each
burst of light will include both a component due to ambient light
and a component caused by the light transmitted through the
patient's body structure from one of the LED's. Assuming that the
amount of ambient light impinging on the photodiode changes slowly,
the amount of ambient light impinging on the photodiode will be
substantially constant during the relatively brief period between
dark intervals. Therefore, the ambient light component of the
photodetector output signal at any time during the period between
dark intervals will be substantially equal to the ambient light
component prevailing during the preceding dark interval and hence
will be equal in magnitude but opposite in sense to the correction
current applied through resistor 80. The correction current will
substantially cancel the component of the photodiode current caused
by ambient light. The net current into preamplifier input node 38
will be substantially equal to the signal component caused by light
transmitted from the LED's. The preamplifier output voltage at node
40 thus will be substantially representative of only the signal
component, and hence will represent only the light transmitted
through the body from whichever LED is illuminated.
Timing means 26 actuates switch 46 to direct the output voltage or
signal from preamplifier output node 40 to the appropriate signal
processing channel, viz., to red signal processing channel 48 while
red LED 16 is illuminated, and to IR signal processing channel 50
while IR LED 18 is illuminated. Each signal processing channel thus
receives a succession of signal samples representing the light
transmitted through the patient's body structure at the associated
wavelength, and hence representing the transmissivity of the body
structure at the particular wavelength. In red signal processing
channel 40 the successive signal samples are smoothed into a
substantially continuous signal by the first low pass filter in
that channel. This continuous signal represents a plot of red light
transmissivity of the body structure versus time. Values
representing that signal are fed by one analog to digital converter
into microprocessor 52. As the AC or varying component of the red
transmissivity signal typically will be small compared to the DC
component, each such value will represent a good approximation of
the DC value. Further, microprocessor 52 performs a digital low
pass filtering to recover a more accurate DC value from successive
digital values. The signal from the first low pass filter within
red signal processing channel 48 is also fed through the high pass
filter in that channel. The high pass filter strips out the DC
component, leaving the AC component which is then amplified and
again low pass filtered to remove residual switching frequency
components and the like. This amplified AC signal is then
successively sampled by the sample and hold device within channel
48 operating under the control of microprocessor 52. Successive
sampled values are fed through a further analog to digital coverter
within channel 48 into the microprocessor, and the microprocessor
52 determines the AC or peak to peak values of the red
transmissivity signal from these successive values. Infrared signal
processing channel 50 coacts with microprocessor 52 in
substantially the same way to recover AC and DC components of the
infrared transmissivity signal. From these transmissivity signals,
microprocessor 52 calculates the level of oxygen or "oxygen
saturation" in the patient's blood and displays that result on
display unit 54. The oxygen level in the patient's blood can be
calculated according to the formulas:
WHERE: ##EQU1## AC.sub.R and DC.sub.R are the AC and DC components,
respectively, of the red transmissivity signal;
AC.sub.IR and DC.sub.IR are the AC and DC components respectively
of the infrared transmissivity signal; and
A, B and C are constants determined by empirical curve fitting in
design of the system, against the results of standard blood oxygen
determinations.
As the transmissivities of the body structure change with the
patient's pulse, the system should be switched between red and
infrared light at a switching frequency greater than the pulse
frequency. Typically, a switching frequency of about 300 Hz is
employed. The dark intervals typically are interspersed with the
alternating bursts of red and infrared light so that a dark
interval follows after each burst or so that a dark interval
follows after every other burst. In the first arrangement, the
sequence of a red burst, a dark interval, an infrared burst and a
further dark interval would constitute one switching cycle, and
this switching cycle is repeated at the switching frequency, viz.,
typically about once every 1/300th second. In the second
arrangement, the sequence of a red light burst, an infrared light
burst and a single dark interval is repeated once on each switching
cycle. With dark intervals provided at rates comparable to the
switching frequency, changes in ambient light and hence changes in
the ambient light component of the photodiode current or signal
between dark intervals caused by factors such as movement of the
patient or of the probe 10 will be insignificant. Where the ambient
lighting includes significant flicker components, typically at
about twice the line frequency or about 100-120 Hz, these flicker
components may induce appreciable ambient light changes between
dark intervals. These changes in ambient light will induce
corresponding changes in photodetector output signal or current
between dark intervals. Inasmuch as the correction current applied
through resistor 80 will not change between dark intervals, the
system does not compensate for these flicker or other rapidly
varying components, and hence these components will be reflected in
the amplified signal appearing at preamplifier output node 40.
However, as in conventional systems, these flicker components are
effectively blocked by low pass filters included in the signal
processing channels. Stated another way, any variation in ambient
light at a frequency comparable to the pulse frequency will be slow
enough that the change between dark intervals is essentially
insignificant. Changes at frequencies comparable to the pulse
frequency will be effectively tracked by corresponding changes in
the correction current during successive dark intervals. Components
of the ambient light changing at frequencies comparable to the
pulse frequency will thus be effectively counteracted by the
correction current and hence will be eliminated by the preamplifier
output signal. Components at higher frequencies, such as the
aforementioned flicker frequency components, will be reflected in
the preamplifier output signal, but these can be segregated from
the useful signals by low pass filtering and hence are not
particularly serious.
As will be appreciated, integrator 62 and inverter 68 and resister
80 cooperatively define a servo feedback loop which effectively
holds the ambient light component or offset the inputs applied to
the preamplifier means input node 38 to zero. Numerous advantages
arise from this approach as compared to prior art systems using a
switched capacitor in the preamplifier output to provide a ground
referenced signal. The capacitor 68 utilized in the preferred
system discussed above can be far smaller than a capacitor required
in a comparable system according to the prior art. This
significantly reduces the capacitive load on the operational
amplifier 36 in preamplification means 34, and therefore
facilitates stable operation of operational amplifier 36.
Operational amplifier 36 is more effectively protected from
overloading caused by ambient light components in the photodetector
output signal. As compared with a prior art system utilizing a
preamplification feedback resistor of the same value and hence
having the same gain, the ability of the system to withstand
ambient light without overloading is increased by a ratio of
R.sub.44 /R.sub.80, where R.sub.44 is the value of resistor 44 and
R.sub.80 is the value of resistor 80. Alternately, the value
R.sub.44 of the feedback resistor in a system according to the
present invention, and hence the gain of preamplification means 34
can be many times greater than the comparable resistor value used
in a system according to the prior art, while still maintaining the
same ability to withstand overloading caused by ambient light.
Typically, systems according to the present invention provide a
combination of increased gain and increased resistance to ambient
light overloads. The currents flowing through switch 56 typically
are smaller than the currents flowing in the switch used in the
outputcapacitor system of the prior art. Resistor 58 is in series
with switch 56, and the value of resistor 58 typically is large in
comparison to the resistance of switch 56. Thus, variations in the
resistance of switch 58 will have relatively little effect on the
response time of integrator 62.
Apparatus according to a further embodiment of the present
invention is shown in FIG. 2. This apparatus includes a photodiode
20', preamplification means 34', integrator switch 56' and
integrator 62' substantially the same as those described above with
reference to FIG. 1. However, in place of the inverter 68 and
correction current resistor 80 utilized in the embodiment of FIG.
1, the apparatus according to FIG. 2 employs a so-called "current
mirror" device 100. The current mirror device includes a source 102
for negative bias voltage and a circuit 103 incorporating two
resistors 104 and 106 connected in series, with a diode 108
therebetween. The circuit 103 is connected between the output node
66' of integrator 62 and negative bias voltage source 102. The
circuit 103 defines a node 110 between the two resistors. A
correction current circuit branch 112 extends from the input node
38' of preamplification means 34' to negative voltage source 102. A
fixed resistor 114 is connected in branch 112, as are the collector
and emitter of a transistor 116. The base of transistor 116 is
connected to node 110 of circuit 103. As will be appreciated, the
impedance across the collector and emitter of transistor 116 and
hence the impedance between node 38' and negative voltage source
102 will vary with the voltage at node 110. Therefore, the
magnitude of correction current flowing through circuit branch 112
will vary with the magnitude of the accumulated signal or voltage
at the integrator output node 66'.
Apparatus according to this embodiment of the present invention
operates similarly to the apparatus described above with reference
to FIG. 1. Here again, during each dark interval, the photodetector
output current from photodetector 20' causes the preamplifier
output voltage at node at 40' to go negative, and hence causes a
progressively increasing positive voltage to appear at integrator
output node 66'. This in turn causes an increasing current through
resistors 104 and 106, and hence an increase in voltage at node
110, leading to a corresponding decrease in the impedance across
the collector and emitter of transistor 116 and hence a
corresponding increase in the correction current on branch 112.
This continues until the correction current equals the photodiode
output current or dark current prevailing during the dark interval
and the system comes to equilibrium. After the dark interval,
switch 56' opens. The value of the integrator output voltage at
node 66' and hence the correction current on branch 112, remain
substantially constant until the next dark interval. Changes in the
impedance of diode 108 with temperature counteract changes in the
characteristics of transistor 116 with temperature.In other
respects, the system operates in generally the same way as that
discussed above with reference to FIG. 1.
As will be appreciated, numerous variations and combinations of the
features described above can be utilized without departing from the
present invention as defined in the claims. For example, the
photodiode 20 or 20' may be reverse-biased, as by a bias voltage
source connected between the photodiode and ground. Also,
photodetectors other than photodiodes may be used. The feedback
loop may be arranged without an integrator. In one such
arrangement, the amplified signal during a dark interval can be
passed to the microprocessor via an analog to digital converter.
The microprocessor may record the value of this signal during the
dark interval and provide this value continually until the next
dark interval to the control input of the correction current
device. The microprocessor would be linked to the timing means to
coordinate this action. Also, where the only parameter of the blood
to be monitored is a pulse parameter, only one signal processing
channel is required, and only one light wavelength is employed. The
microprocessor may also perform other, conventional functions such
as controlling the gain of the amplifiers in the signal processing
channels and the power applied to the LED drives to keep the
signals supplied to the various analog to digital converters in
range. As these and other variations and combinations can be
employed, the foregoing description of the preferred embodiments
should be taken by way of illustration rather than by way of
limitation of the present invention as defined by the claims.
* * * * *